The invention concerns the preparation of azetidinones useful as intermediates for preparing penems and carbapenems.
Carbapenems (I) and penems (II) are classes of antibiotic compounds characterized by the formulae ##STR1##
Compounds of special interests are I or II when the substituent CH.sub.3 --CHOH-- is in the six position and has a particular isomer configuration. An example of such a compound is thienamycin having the formula ##STR2## A useful synthesis of a compound such as A requires an intermediate azetidinone of the formula ##STR3##
Introduction of the CH.sub.3 --CHOH--side chain via alkylation of the .beta.-lactam enolate is a direct route to B, however, it suffers from being nonstereospecific. A less direct route to B is a two step sequence involving acetylation of a .beta.-lactam enolate followed by stereospecific reduction of the .beta.-keto lactam product. This reduction requires sophisticated and expensive reducing agents.
The present novel process has been discovered which provides a simpler and more direct route to the B intermediate protected as an O-silyl ether.